Before we can underſtand the nature of earths, we muſt know their component parts. Thoſe earths which cannot be further decompoſed we call primitive, and thoſe which conſiſt of two or more of theſe intimately united, derivative. By this union we do not mean a mere mechanical diffuſion, at leaſt not ſuch as can be diſtinguiſhed by the eye, as is the caſe in ſtones, (ſaxa.)
It is evident that the primitive earths will conſtitute ſo many natural Genera, and different mixtures of theſe the Species.
They who would make ſeveral Genera out of one primitive earth, muſt ſeparate the glaſſy, red, white, horny ſilver ores, and other different compoſitions into as many Genera, or elſe act inconſiſtently with their own principles.
At preſent we only know five primitive earths. They who reckon fewer, reſt their opinions upon fanciful metamorphoſes unſupported by faithful experiments[37]. As experiments teach us that there are five primitive earths, it is evident that the Species ariſing from the mixture of theſe cannot exceed twenty-four, viz. 10 double (conſiſting of two earths) 6 triple, 3 quadruple, and the 5 primitive.
Although theſe different mixtures are poſſible, and probably do exiſt, they have not yet been all found. The natural compoſitions of acids with the earths, forming ſubſtances not ſoluble in 1000 times their weight of boiling water, and which may be called ſaline earths, muſt be added to the ſpecies, as they are certainly chemical combinations.
The primitive earths hitherto detected are,
| TERRA PONDEROSA, or | heavy earth. |
| CALX, | calcareous earth. |
| MAGNESIA, | magneſia. |
| ARGILLA, | argillaceous earth. |
| TERRA SILICEA, | ſiliceous earth. |
And we muſt believe theſe to be primitive, until it ſhall appear by proper experiments that they may be ſeparated into others ſtill more ſimple, or changed into one another by art.
Theſe are firſt to be conſidered in their greateſt ſimplicity and purity, although nature never preſents us with ſuch, nor can they even by art be rendered abſolutely free from all heterogeneous mixture. Water and aerial acid readily unite with the four firſt, and when expelled by fire, a little of the matter of heat is added, and remains until driven out by a more powerful attraction. But in this ſtate they poſſeſs a degree of purity not to be attained by any other known method. Therefore it is neceſſary to examine them when ſufficiently burnt in order to diſtinguiſh better what properties depend upon adhering heterogeneous matters.